During downhole drilling operations, mud and drilling fluids are circulated within the wellbore by being pumped down through the drill string and returning to the surface via the borehole annulus. Drill cuttings produced during drilling are carried up to the surface through the annulus by the drilling mud. However, in extended reach wells and/or highly deviated or slim diameter wells, the pressure of the drilling mud along the circulation path can drop from that at the surface, which results in a lower cutting lifting performance which in turn can lead to restrictions/obstructions arising in the annulus caused by accumulating cuttings.
In order to alleviate this problem, it is conventional to include one or more downhole circulating subs in the drill string which allow fluid circulation rates to be varied by selectively opening a path from the interior of the drill string to the annulus. Ports in the circulating subs can be opened and closed to enable the flow path of drilling fluids to take a different course, thereby altering the circulation time.
Conventional circulating subs typically comprise a ball seat and, in the event of a restriction in the circulation path at a location in the annulus above that of the circulating sub, a ball, of greater diameter than the seat at its narrowest point, is dropped or pumped through the drill string such that it lands on the ball seat. Once in position, the area above the ball and ball seat becomes sufficiently pressurised to move the ball seat downwards thereby uncovering the ports which enables the drilling fluids to flow through ports in the sidewall of the circulating sub and string into the annulus.
Typically, a series of circulating subs is provided within the drill string at vertically spaced apart points. In view of the method of operation of the ball seats, vertically higher ball seats necessarily have a greater inner diameter than vertically lower ball seats allowing smaller balls destined for the lower seats to bypass higher circulating subs when dropped downhole. Due to the progressively narrower inner diameter required towards the bottom of the casing, a drill string can usually only accommodate a maximum of six such circulating subs.
The aim of the present invention is to provide an improved circulation sub and an improved method of actuating downhole tools which alleviates problems associated with the prior art described hereinbefore and also provides a means of sending instructions and/or data from/to downhole tools.